[low prio FR] add `wupget:AllowBuildGrid`

`wupget:AllowBuildingGrid(wantedX, wantedZ, unitDefID, options) -> cappedX, cappedZ`.

Say you drag a 7x7 box of buildings. So the function receives 7, 7 as the input. (options are like CTRL or ALT which make the box behave differently)
And you return 4, 5. Now the drag behaves as if you only dragged that far:

Note that the function is unsynced and happens continuously as you adjust the drag (perhaps only if the numbers/options change and not actually continuously, but both seem fine), and the built-in UI reflects the resulting grid.

The main practical purpose for existing games would be to limit accidental giant orders that lag everybody, but it seems generally useful (for example if somebody made a starcraft style game they could limit barracks grids to 2xN because else the middle row has some completely surrounded buildings with no exit).

Perhaps there would also be a version for line-build? Unsure what interface that one would take.

No file or test is named; start by locating the building-grid handling and the existing wupget API entry points. Define the AllowBuildingGrid interface and verify that the built-in UI uses its capped dimensions during drag operations, while deciding whether line-build support is in scope.

Written by the indexing model from the issue text.